The conventional procedure for the transmission of two independent FSK channels is to employ two independent lasers and modulate them individually to obtain two binary channels. The detection scheme can be either optical discrimination or by heterodyne detection. As two lasers are employed, both channels can be effectively on at the same time. Normally the use of one laser to transmit two independent data streams is precluded unless the symbol rate of the laser is increased, or a multilevel signal is transmitted to a complicated receiver.
An object of the invention is to provide apparatus allowing transmission of two independent FSK channels by one laser transmitting at the same symbol rate by appropriate coding and reception of one or both such channels by relatively simple coherent detection.